1. Field of the Invention
This invention relates to a high pressure-low pressure single cylinder turbine rotor and a method for the production thereof.
2. Description of the Related Art
Generally in a steam turbine, rotors of materials differing with working conditions of steam are used as combined. One example of the conventional steam turbine is shown in FIG. 6. In a large steam turbine, for example, a CrMoV steel exhibiting outstanding creep rupture strength at high temperatures is used as the material for the rotor operating on the high temperature-high pressure side (near 566.degree. C., for example) 1 as specified by ASTM-A470 (Class 8). For the rotor operating on the low pressure side (at or below 350.degree. C., for example), a NiCrMoV steel having a Ni content not less than 2.5% is used as specified in ASTM-A470 (Class 2 to 7). Then, these rotors which are made of materials differing with steam conditions are mechanically joined at a junction 3 to construct a steam turbine serving to rotate an electric generator 4. Since the conventional large steam turbine is constructed by joining a plurality of rotors made of different materials as described above, it has the drawback that the process of manufacture is complicated, the floor space for the installation of the turbine proper as a whole is large, and the cost of the plant is inevitably great.
In contrast, in a relatively small steam turbine (a power generating plant of an output of not more than 100 MW), a high pressure-low pressure single cylinder rotor made of one and the same material is generally used as extended from the high pressure side through the low pressure side. As the material for the conventional high pressure-low pressure single cylinder rotor, a CrMoV steel, a NiCrMoV steel, and a 1CrMoVNiNb steel are generally used.
In a power generating plant having an output exceeding 100 MW, however, the use of a steam turbine incorporating a high pressure-low pressure single cylinder rotor entrains the following problem. One example of the steam turbine using a high pressure-low pressure single cylinder rotor is shown in FIG. 7. With reference to FIG. 7, the high pressure part 1 of the steam turbine rotor is used in an environment of high temperature exceeding 500.degree. C. and the downstream side portion of the high pressure part 1 and the upstream side portion of the low pressure part 2 are used in a temperature range of from 350.degree. to 450.degree. C. The CrMoV steel heretofore used as the material for the high pressure-low pressure single cylinder rotor, therefore, is not fully satisfactory in terms of tensile strength and toughness. Though the NiCrMoV steel excels in tensile strength, it nevertheless has the problem that it is deficient in creep rupture strength and is liable to succumb to embrittlement in a temperature range exceeding 350.degree. C. Then, the 1CrMoVNiNb steel is hardly satisfactory in terms of tensile strength and toughness. A 12Cr steel has been already developed as the rotor material excelling in creep rupture strength and toughness and in tensile strength in a low temperature range as well. Since this 12Cr steel is expensive, the use of this alloy as the material for a rotor entrains the problem of increasing the cost of production. The circumstance has urged development of an alloy which incorporates W besides Ni, Cr, Mo, V, or the like and further incorporates B and N (JP-A-63-157,839).
A high pressure-low pressure single cylinder turbine rotor which is capable of retaining such creep rupture strength as is required in a high pressure part and meanwhile repressing the loss of strengths (tensile strength and creep rupture strength) due to aging and also capable of retaining such toughness and tensile strength as are required in a low pressure part and meanwhile repressing the decline of toughness (embrittlement) due to aging and consequently usable at a power generating plant of an output exceeding 100 MW remains yet to be developed.